i want to replace my 2 thermostat (1988 xjs 5.3l)
is there a "right" way to do it ?

 

You have to make sure that you get the right ones; they have a 'jiggle' pin that sits at the 12 o'clock position and allows air to pass through when you bleed the system.
You must bleed it properly otherwise you will get air locks which will damage the engine when it overheats.
You need to download the Kirby Palm XJS book which is the ultimate reference point for all things XJS related.

 

^ Otherwise, no not really. Just have the right t-stats and put them in correctly.

You just pull some hoses and unbolt the front of the housing and that is about it. You can buy some gaskets when you order the t-stat but you could also just cut your own if you have gasket paper laying around. Remember the housing is aluminum, no need to overtighten anything. IIRC the spec is something like 14 ft-lbs, (too low to use a 3/8 drive torque wrench) but that still gives you some idea not to overdo it.

Probably the most tedious part is just cleaning the gasket surfaces, otherwise its a pretty straight forward job.

 

Nothing unique to the jag. +1 on jiggly pin. Make sure both gasket surfaces are totally free of debris and build up tightness on screws going back and forth across the housing. Don't over tighten and ruin your day..